Electrostatic recording process



May 25, 1965 J. E. DICKENS ELECTROSTATI C RECORDING PROGES S Filed March 5, 1962 INVENTOR JOHN E. DICKENS ORNEY United States Patent 3,185,995 ELECTROSTATIC RECORDING PROCESS John E. Dickens, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Filed Mar. 5, 1962, Ser. No. 177,621 Claims. (Cl. 346-1) This invention relates to the process of electrostatic recording.

In the electrostatic recording process, a dielectric recording medium travels between a pair of electrodes. As the voltage between the electrodes is increased, a threshold value of the voltage is reached at which current flow in the circuit begins to increase rapidly. At voltages above the threshold value there is a steady flow of current of substantial magnitude which continues so long as the recording medium continues to move past the electrodes. This voltage above the threshold value serves to inject into the record medium a charge pattern corresponding to a signal. This pattern cannot be removed by wiping the surface of the medium. Furthermore, the signal can be played back repeatedly by coupling the electrodes to an amplifier and passing the record medium between the electrodes. For dielectric materials such as Mylar polyester tape having a thickness of 0.25 mil, the threshold voltage is about 600 volts.

In the specific operation of the process, the dielectric medium in the form of a high resistivity tape is passed between two electrodes. The electrodes contact opposite surfaces of the tape to make the tape part of the electrical circuit. A uniform bias voltage of 600-1200 volts (DC. or AC.) and :a Voltage corresponding to the signal to be recorded is impressed across the thickness of the moving tape by the electrodes The preferred electrode structure comprises a knife edge frontal electrode and a discontinuous backing electrode. The preferred backing electrode is a wire-wound cylindrical structure having a multiplicity of fine point contacts transverse to the direction of movement of the tape and directly opposite to the knife edge. These fine point contacts in combination with the opposed knife edge provide sites of high electric field intensity required forpermanent recording.

The ability of the dielectric medium to retain the signal is a function of the volume and surface resistivities of the medium. For substantially permanent retention, a volume resistivity of at least 10 ohm-centimeter is required. Although there are many materials that fulfill this requirement, Mylar* polyester tape fulfills both this requirement and the inherent requirement of adequate strength. The strength is required since the tape must travel many times through the recording and playback apparatus.

Electrostatic recording on an electrically non-conductive dielectric medium as discussed above is potentially superior to systems of recording in common use. This superiority is based upon the low cost of the recording medium; the low cost of the accessory apparatus; and the higher information packing density possible. General acceptance of this method of recording, however, has been retarded by certain deficiencies. One of these deficiencies is associated with the apparatus and method employed in recording. This apparatus, by virtue of the requirement for heavy cont-act of electrodes with the moving dielectric tape, exerts a vise-like action on the tape while the tape passes between and in engagement with the relatively sharp frontal electrode and the blunt backing electrode. This causes abrasion of the surface of the tape with the accompanying build-up of debris on the electrode system. This debris causes erratic contact and poor quality recording. A second deficiency of electrostatic recording is the *Registered trademark of E. I. du Pont de Nemours & Co.

Patented May 25, 1965 difficulty to store recorded tape. There is a tendency of recorded signals to print through from one convolution to the next if the recorded tape is wound on a reel for storage.

The present invention overcomes these difiiculties by using as the recording medium a dielectric material in the form of a continuous tape or film having one surface thereof coated with an electrically conductive material. The coated dielectric material is preferably metallized polyethylene terephthalate film. However, other polymeric films may be used as the dielectric material, e.g., polyethylene, polystyrene, polypropylene, fluorinated olefin polymers, etc. Metallizing of the dielectric material may be accomplished using metal vapors in the conventional manner to provide a coating of aluminum, copper, tin, gold, etc. It is also possible to coat the dielectric film with electrically conductive particles dispersed in a vehicle. Besides metal particles, e.g., aluminum, copper, etc, non-metal particles such as carbon, silicon, etc., particles may be used as well as metal oxide particles, e.g., tin oxide.

The use of the electrically conductive backing on the dielectric recording tape permits recording with very light contact and, in many cases, without any substantial contact at all between the dielectric tape and the electrodes. In fact, it is preferable to omit the backing electrode entirely and permit one of the drive rollers, the capstan, not in opposed relationship with the recording electrode to act as a grounded backing electrode. Surprisingly, also, the signals recorded in a dielectric medium with a conductive backing persist for satisfactorily long periods even when storage of the record medium is in a relatively moist atmosphere. It should also be pointed out that are formation over the edge of the dielectric tape between the recording electrode and the conductive backing during recording, envisaged by those skilled in the art for such recording, does not occur.

The invention will be further clarified by referring to the drawing, in which:

FIGURE 1 is a schematic illustration of an apparatus useful for performing the process of this invention.

Referring to the figure, the recording medium 11 in the form of a tape having :a metal-lized backing, preferably .alumi-nized polyethylene terephthalate tape, is passed from supply reel 12 to windup reel '13, driven by engagement between the driven capstan -14 (driven by a motor not shown) and idler roll 15. The frontal electrode system 16 is shown to be disposed below the traveling recording medium. This frontal electrode system 16 includes sources for direct current biasing and for providing a signal. This system 16 may be composed of one or two or more electrodes. Although contact of the electrode with the record medium can occur, a spacing of 0.1 mil- 3 mils is usually maintained between the medium 11 and the electrodes or electrode. The preferred maximum spacing is 0.3 mil for recording using DC. voltage and 0.5 mil for recording using radio frequency AC. voltage. The recording electrode is usually coupled to a cathodefollower type preamplifier 21, such as employed in a Kiethley 102-13 electrometer (manufactured by Kiethley Electronics (10., Cleveland, Ohio).

If desired, the recording medium may travel adjacent to ion sources 18 and 19. These ion sources are of the high voltage discharge type and are coupled to a high voltage transformer 20. Ion source 18 serves to condition the tape prior to recording and ion source 19 serves to lock the signal in the tape after recording.

No backing elect-rode opposite the frontal pre-bias electrode and/or the recording knife edge electrode is necessary. The frontal electrodes should not be as Wide as the tape but only of half-track width. In this way, arc-over is avoided.

Recording is accomplished by using a pre-bias of 1000 volts DC. applied to the first electrode, with a recording bias of +600 volts D.C. along with a signal having a maximum of 600 volts peak-to-peak applied to the second electrode. (The negative sign applied to the pre-bias voltage indicates that the negative pole of the bias source was applied to this electrode and, correspondingly, the positive sign on the recording bias voltage indicates that the positive pole of the electrode is coupled to this electrode.) it is to be understood that recording could be accomplished on apparatus with only a single frontal electrode, in which case the tape would be passed through the apparatus first and subjected to :the pre-bias voltage, and then through the apparatus a second time for recording. Likewise, recording could be accomplished with a single electrode without pre-biasing. However, a higher voltage (about 1200 volts) bias is required.

The structure of the frontal electrodes is preferably a knife edge of about one-half the tape width in length, with the edge transverse to the tape between shoulders of a high-strength, high-resistivity resin such as Delrin. These shoulders extend over the edges of the tape and provide guides to assist in uniform tracking the electrode. Playback is in the usual manner, although slightly improved results can be obtained with a playback head of a width narrower than the tape.

Recordings were made at selected frequencies over the range from 50 c.p.s. to 10,000 c.p.s. These recordings wound on reels, were found to have persisted without serious deterioration for periods comparable to those recordings on uncoated polyethylene terephthalate recorded by recording methods involving contact of the electrodes with the medium. After storage for sixty days the extent of decay was comparable, with no print-through of the recording on the metallized tape. No significant build-up of insulating debris on the knife edge electrode was observed in recording on the metal-lized tape.

Although only audio recording has been described, it should be understood that this invention applies to video and other analog recording, digital recording, etc. and, in short, to electrostatic recording of all types.

What is claimed is:

1. In the process of electrostatic recording wherein a dielectric recording tape is passed between a pair of electrodes, a frontal recording electrode and a backing elec- 'Rcgistered trademark of E. I. du Pont de Nemours 8; C0.

trode, and is subjected to a voltage across the electrodes, said voltage being above the threshold voltage for current flow to said dielectric tape and also being directly related to a signal voltage whereby a charge is impressed in said dielectric tape corresponding :to the signal, the improvement wherein the surface of the dielectric tape distant from said frontal recording electrode is electrically conductive.

2. A process as in claim 1 wherein said dielectric recording tape is in contact with said backing electrode.

3. A process as in claim 2 wherein said backing electrode is a drive roller.

4. A process as in claim 1 wherein one surface of said tape is in proximity to but out of contact with said recording electrode during the recording process.

5. A process as in claim 1 wherein both surfaces of said tape are in proximity to but out of contact with said recording and backing electrodes during the recording process.

6. A process as in claim 1 wherein the conductive surface of said dielectric recording tape is a metal coating on said tape.

7. A process as in claim 6 wherein said metal is aluminum.

8. A process as in claim 1 wherein the dielectric recording tape is polyethylene terephthalate film.

9. A process as in claim 1 wherein the dielectric recording tape is polyethylene terephthalate film which is coated with a metal.

10. A process as in claim 1 wherein the dielectric recording tape is polyethylene terephthalate film which is coated with aluminum.

References Cited by the Examiner UNITED STATES PATENTS 2,698,928 1/55 Pulvari 340-173 2,726,940 12/55 Buhler -17 3,012,839 12/61 Epstein et al 346-74 3,040,124 6/62 Camras 1786 FOREIGN PATENTS 896,692 5/62 Great Britain.

LEO SMILOW, Primary Examiner.

LEY LAND M. MARTIN, Examiner.

a1 judgl 1d 4.

J. E. Dickens, nent adverse to 

1. IN THE PROCESS OF ELECTROSTATIC RECORDING WHEREIN A DIELECTRIC RECORDING TAPE IS PASSED BETWEEN A PAIR OF ELECTRODES, A FRONTAL RECORDING ELECTRODE AND A BACKING ELECTRODE, AND IS SUBJECTED TO A VOLTAGE ACROSS THE ELECTRODES, SAID VOLTAGE BEING ABOVE THE THRESHOLD VOLTAGE FOR CURRENT FLOW TO SAID DIELECTRIC TAPE AND ALSO BEING DIRECTLY RELATED TO A SIGNAL VOLTAGE WHEREBY A CHARGE IS IMPRESSED IN SAID DIELECTRIC TAPE CORRESPONDING TO THE SIGNAL, THE IMPROVEMENT WHEREIN THE SURFACE OF THE DIELECTRIC TAPE DISTANT FROM SAID FRONTAL RECORDING ELECTRODE IS ELECTRICALLY CONDUCTIVE. 